Pressure-stabilized polymerization of nitrogen in manganese nitrides at ambient and high pressures.

Two stable high-pressure phases ( C 2/ m -MnN 4 and P 1̄-MnN 4 ) and four metastable phases ( P 4/ mmm -MnN 4 , P 1̄-MnN 5 , C 2/ m -MnN 6 and P 1̄-MnN 8 ) are proposed by using ab initio evolutionary simulations. Besides the reported quasi-diatomic molecule N 2 , the armchair chain and S-like chain, the N 4 ring and N 22 ring are firstly reported in the P 4/ mmm -MnN 4 and P 1̄-MnN 5 phases. A detailed study is performed on the energetic properties, mechanical properties and stability of these polynitrogen structures. Ab initio molecular dynamics simulations show that P 1̄-MnN 4 and P 1̄-MnN 5 can be quenched down to ambient conditions, and large decomposition energy barriers result in the high decomposition temperatures of P 1̄-MnN 4 (2000 K) and P 1̄-MnN 5 (3000 K). Interestingly, P 4/ mmm -MnN 4 with the N 4 ring exhibits outstanding mechanical properties, including high incompressibility, high hardness, uniform strength in the 2-D direction and excellent ductility. Strong N-N covalent bond and weak Mn-N ionic bond interactions are observed in the predicted Mn-N compounds, and the charge transfer between the Mn and N atoms provides an important contribution to the stabilization of polymeric N-structures. All the proposed structures are metallic phases. Our results provide a deep understanding of the chemistry of transition metal polynitrides under pressure and encourage experimental synthesis of these new manganese polynitrides in future.